Getter structure



June 3, 1958:

Filed Dec. 29, 1954 l. S- SOLET El' AL GETTER STRUCTURE 2 Sheets-Sheet 1IN VEN-TORJ HUBERT L .'WHER Ina S. SDLET rah/iv June 3, 1958 vl. s.soLET ETAL INVENTORS HUBER-.r I .WHER a l IRR S. SDLET www i and to animproved method of making the same.

United States GETTER STRUCTURE Application December 29, 1954, Serial No.478,352

Claims. (Cl. 2G6.4)

This invention relates to an improved getter structure Moreparticularly, the invention provides an improved getter wire having acore containing a material which exhibits gettering properties and asheath surrounding the core and provides an improved method of makingsuch a wire. The wire made according to the method of the inventionproves useful as a getter, that is, as a clean-up agent for removingresidual gases within an evacuated electron discharge device. Thestructure of the invention protects the core of the wire from anatmosphere which is chemically reactive with the material of the core.The method vof making the structure includes the casting of a corematerial in a container.

Metals which prove ideally suited as getter materials, due to theirrelatively high chemical reactivity with gases, are relatively diiiicultto handle because they also react rapidly with moisture and some of thegases in the atmosphere. Barium, for example, forms barium oxide andhydrated oxides on exposure to the atmosphere.

Due to the oxidizability of such highly reactive metals, these metalshave heretofore been introduced into electron tubes as alloys. However,unless suchalloys contain a relatively small percentage of the highlyreactive getter material, they are still attacked by the oxygen andwater vapor of the atmosphere. Due to the necessary high percentage ofalloying stable material, a relatively high temperature is usuallyneeded to liberate the getter material. Even at high temperatures thegetter material is given oi relatively slowly. Protracted hightemperature treatment of the alloy accompanied by vapoi-ization ofundesired constituents may be deleterious. For example, when analuminum-barium alloy which is stable in the atmosphere is introduce-dinto an electron tube the alloy is decomposed and the barium liberatedonly at a higher temperature than that needed to activate or flash thebarium. By reason of the higher heating required there is also the riskthat the heating may be carried so high that although aluminum has ahigher temperature of volatilization than barium, part of the aluminumvolatilizes and deposits on those parts of the tube where the barium isto be precipitated.

Space limitations often permit only a limited amount of getter materialwithin an electron tube. The need for a relatively large amount ofalloying stable material reduces the yield of the reactive materialwithin the tube. In some cases the yield has been inadequate for athorough gettering action.

Previous attempts to mechanically encase a core of a getter material, ina sheath have resulted in the introduction of gases between the outsidesurface of the core material and the inside surface of the sheath. Whensuch a structure is activated within an evacuated tube for -getteringpurposes, the entrapped gases are released. This increases the amount ofgas within the tube.

An object of the invention is to provide an improved atent ICC structurecomprising a container having therein an atrnospherically reactivematerial.

A further object is to provide a structure comprising an atmosphericallyreactive alkaline earth metal and a container shielding said metal fromthe atmosphere and which is free of entrapped gases.

It is another object of the invention to provide an improved getterstructure having a core of barium and a sheath of aluminum and whereinsaid core completely lills the space within the sheath so that theregion between the outer surface of the core and inner surface of thesheath is free of entrapped gases.

It is a further object of the invention to provide an improved method ofcasting, into a container, a material which has a melting point abovethat of the container and which is reactive with the atmosphere.

A still further object of the invention is to provide an improved methodof making a getter structure for use Within an electron tube and whereina core of an alkaline earth metal is encased within a sheath of aluminumwithout the entrapment of gases within the sheath.

It is yet another object of the invention to provide an improvedaluminum sheathed wire comprised substantially of an alkaline earthmetal and Which is adapted to be used as a getter material within anelectron tube.

According to the invention a structure and a method of making the sameare provided for attaining the foregoing objects.

While the invention is pointed out with particularity in the appendedclaims, it may be best understood from the following detaileddescription and drawing wherein like numerals refer to like parts. Theembodiments described are presented solely for illustrative purposes andnot by way of limitation.

In the drawings:

Figure l is a flow chart of a method of making an aluminum-clad alkalineearth metal structure according to the invention.

Figure 2 is a liow chart showing in greater detail the steps of a methodof the invention.

Figure 3 is a vertical cross-sectional view of a mold assemblyillustrating a casting step according to the invention.

Figure 4 is a cross-sectional view taken on line 4-4 of the moldassembly shown in Figure l.

Figure 5 is a side view partly in section of a structure produced by themold apparatus shown in Figure 3.

Figure 6 shows an enlarged perspective view partly in section of analuminum sheathed barium wire made according to the method of theinvention.

Referring now to the drawings in greater detail there is shown in Figurel a flow chart of a method, according to the invention, which is used inmaking an aluminum-clad alkaline earth metal structure. The material tobe used for the core of the structure, which includes a metal of thealkaline earth group and which may have a melting point above that ofaluminum, is melted and poured into an aluminum container. The aluminumcontainer is maintained at a temperature below its melting point whilethe core material in the container cools. The core material completelyfills the space within the container by virtue of its being poured intothe container; consequently, the resultant structure is substantiallyfree of entrapped gases. The operations or steps shown in the flow chartof Figure l will be described in greater detail below in connection witha ydescription of one embodiment of the invention.

Figure 2 depicts an embodiment of the method illustrated in Figure l. Avacuum melting furnace (not shown) may be used to melt the corematerial. The core material used in this embodiment of the invention iscomposed of an alkaline earth in metal form such as barium having apurity of the order of 99 barium. The core material is melted undervacuum or in an atmosphere of an inert gas at a relatively low pressureand at a temperature of above 850 C., the melting point of barium. Forexample, at atmosphere of relatively pure argon at a pressure of about0.2 atmosphere may be used. The vCrucible used in containing the bariumduring the melting operation may be of a material known as Armco Iron. Athermocouple gauge (not shown) may be used for the reading of relativelylow pressures and a mercury manometer (not shown) may be used forreading higher pressures.

The melting may be accomplished by first placing the barium metal withinthe crucible in the chamber' of a vacuum furnace and evacuating thelatter to a relatively low pressure, say 25 microns of mercury. Thebarium is then slowly heated until most of the resultant gas evolutionceases. The chamber is then flushed out a number of times with an inertgas such as argon to remove substantially all traces of oxygen; twoflushings have proven sulicient. The pressure of argon is then adjustedto about 100 microns of mercury so as to provide a vapor pressure ofargon which is at least as great as the vapor pressure of barium at itsmelting point to lprevent the barium from boiling off into the furnace.The temperature of the furnace is then increased to melt the barium.After the barium is completely molten the heat is increased and the meltis held at an elevated temperature for about five minutes in order todegas the melt and to achieved an elevated pouring temperature. The meltis then poured, in an inert atmosphere, into a mold assembly of the typeshown in Figure 3; the mold assembly is then allowed to cool to roomtemperature in the same inert atmosphere.

There is illustrated in Figure 3 an apparatus showing the position ofthe core material and the aluminum sheath or container 12 within a moldassembly 14. The mold assembly 14, which may comprise a copper moldsupport 16 closed off at the bottom thereof with a suitable metal plug18, is relatively massive compared to the aluminum and barium materialwithin the support. A copper mold support having a length of 71A inchesand a Wall thickness of one inch has been used. The support may besplit, as shown in Figure 4, to facilitate removal of the compositestructure produced by the mold assembly. While the use of a split coppermold support is preferred, a mold support of any other material may beused having a thermal conductivity and capacity such that thetemperature of the inside surface of the container 7 is maintained at atemperature below the melting point of aluminum throughout the pouringand cooling steps.

A funnel 20, which may be of a material such as graphite so as to reduceany alkaline earth metal oxides that may be formed, may be disposedaround an opening at the top of the mold assembly as viewed in Figure 3inorder to direct the ow of barium into the aluminum container 12. Thealuminum container 12 may have an inside diameter of about one-half inchand a wall thickness of the order of three sixty-fourths of an inch.

It will be noted from the foregoing that an inside diameter of one-halfinch is used. It has been found that if the inside diameter of thealuminum container is reduced to appreciably below one-half inch thepoured material is cooled to solid state before it reaches the bottom ofthe container and prevents the formation of the desired compositestructure. If the inside diameter of the aluminum container is increasedappreciably beyond onehalf inch the larger mass of poured material willpossess a magnitude of heat such that special cooling means are neededtomaintain the inside surface of the aluminum container below its meltingpoint. Such cooling means are relatively expensive. K

After the core material has cooled to room temperature, the structureproduced by the mold is removed. Figure 4 5 is a view partly in sectionof such a structure. The ends of the aluminum container 12 are pinchedtogether in order to seal the core material 10 from the atmosphere.

The structure may be drawn down to the desired diameter by means ofwire-drawing dies. Since the aluminurn container is relatively ductileany spaces formed between the outside surface of the core material andthe inside surface of the aluminum container are substantiallyeliminated in the drawing operation by the pressure forcing the aluminumsheath against the core. The Wire thus formed may then be cut into thedesired lengths by means of a pinching operation so that an aluminumcoating is retained around the barium'at the severed ends. Thus, it isseen that barium, which has a melting point of about 850 C., may be castinto an aluminum container which has a melting point of about 660 C.,without melting the container. Since aluminum exhibits a boiling pointof about 1140a C., and barium exhibits a boiling point of about 2056 C.,barium will boil off or flash at a lower temperature than aluminum.Therefore, barium rather than aluminum will be more likely to bedeposited on surfaces within an electron discharge device for providinggettering action within the device. Similarly, other metals of thealkaline earth group, namely, strontium-melting point of about 800 C.,calciummelting point of about `810" C., and magnesium-melting point ofabout 651 C., may be cast in an aluminum container which has a meltingpoint which is lower than that of the melt which is cast into thecontainer. When magnesium is used as the casting material, the melt isordinarily heated to a temperature substantially above 660 C., themelting point of aluminum, in order to lassure a free llow of the meltinto the container.

While the method of the invention has been described with regard torelatively pure alkaline earth metal cores, the method of the inventionmay be used in the casting of alloys containing a metalfrom. the groupof alkaline earth metals. For example, it is often desired touse abarium-aluminum alloy as a getter material for certain high temperatureflash getters wherein the barium-aluminum alloy used is relativelyunstablein air. it is often desirable to alloy a relatively smallquantity of aluminum with an alkaline earth metal getter material inorder to improve the workability of the. getter material so that a slugof the getter material maybe more easily rolled, swaged, or drawnthrough wire-forming dies to produce getter material in the form ofrelatively thin wire.

The barium-aluminum alloy may be cast by the method of the invention toproduce an aluminum-clad bariumaluminum alloy core structure. One gettermaterial made according to the method of the inventionv has a core of analloy of barium of 99% purity and aluminum of 99.6% purity in a ratio of99.0 grams of barium to 1.0 gram aluminum i. e. 99% barium and 1%aluminum by weight. The barium may be weighed in paraine oil and rinsedin toluene before being placed in the melting Crucible with thealuminum. The melting crucible is placed in a vacuum melting furnace ofthe afore-described type and the vacuum chamber is evacuated to arelatively low pressure so as to remove barium reactive gases. Thecharge is then slowly heated until the evolution of gases ceases and thechamber flushed out with argon. The pressure of argon is then adjustedto about 150 millimeters of mercury and the charge quickly melted.

An alternative method of removing the 4barium reactive gases may beused. In the alternate method the barium v is deliberately allowed toreact with the residual gases in Then, too,

5 the increased temperature in order to degas the melt and to achieve anelevated pouring temperature. is then poured into the `aluminumcontainer in the mold assembly of the aforedescribed type and allowed tocool to room temperature in the argon atmosphere.

Instead of the argon atmosphere called for in the above description anyother inert atmosphere may be used provided the gas of the inertatmosphere is not absorbed by the core material or the aluminum liner.For example, helium or neon may be used as the inert atmosphere.However, argon is preferred for reasons of economy. In stead of theinert atmosphere a vacuum maybe used; but, as mentioned before, the useof a vacuum is not preferred.

Figure 6 shows a portion of a wire adapted to be used as a gettermaterial within an electron tube and which was made according to themethod of the invention. While the drawing shows a sectional view of thewire for purposesy of illustrating its structure, the core material 22,which is of a material including an alkaline earth metal, is preferablycompletely sheathed by a coating of aluminum 24. l

While the aluminum-clad structure made according to the method of theinvention is useful as -a getter material within electron tubes, it willbe appreciated that the invention is equally useful in other applicationwhere a core of a highly reactive material is desired which issubstantially free of entrapped gases.

What is claimed is:

1. A structure adapted to be used as a getter within an electrondischarge device comprising a sheath of a material including aluminum,and a solid core member including barium within said sheath, said coremember completely lling the space within said sheath.

2. A structure adapted to be used as a getter within an electrondischarge device comprising a solid core member including an alloyconsisting of aluminum and at least 50% barium by weight, `and a sheathof aluminum surrounding said core member, said core member completelyfilling the space within said sheath.

3. A structure adapted to be used as a getter within an electrondischarge device comprising a solid core member including an alloycontaining barium and aluminum in the ratio of about 99% barium to about1% aluminum by weight, and a sheath of aluminum surrounding said coremember, said core member completely filling the space within saidsheath.

4. A method of making an aluminum-clad core structure having a corematerial including a metal selected from the class consisting ofalkaline earth metals and alkaline earth metal alloys and characterizedin being reactive with the ordinary atmosphere, and comprising theoperations of heating said material to a temperature below the meltingpoint thereof and in an atmosphere at below ordinary atmosphericpressure whereby said material is substantially degassed, furtherheating said material in an atmosphere of an inert gas at a pressure atleast as high as that of the vapor pressure of said material and Thealloy to a temperature sufficient to melt said material whereby saidmaterial is melted while evaporation of said material is reduced, stillfurther heating said material in said last named atmosphere forachieving a pouring temperature of said material which is above ythemelting pointthereof, pouring said material in an inert gas at said lastnamed pressure into an aluminum container which has a melting pointbelow said melting point while drawing heat from the inside surface ofsaid container through said container to maintain said inside surface ata temperature below the melting point thereof and thereby preventing anyalloying between said core material and said container, cooling saidmaterial in an inert gas to a temperature below the melting point ofsaid container, and mechanically working said container to remove anygases entrapped therein with said core material and to `close an openingin said container, whereby an aluminum-clad core structure is providedhaving a substantial freedom from entrapped gases.

5. A method of making aluminum-clad wire having a core material whereinthe major constituent is barium, and comprising the operations ofheating said material to a temperature below the melting point thereofand in an atmosphere at a pressure below that of the ordinary atmospherefor degassing said material, further heating said material in anatmosphere of argon at a pressure of at least 0.2 atmosphere wherebyevaporation of said barium is reduced, raising the temperature of saidmaterial to pouring temperature within said atmosphere of argon, pouringsaid material into a mold assembly including an aluminum container andin said atmosphere of argon, said mold assembly being characterized by athermal conductivity andcapacity sucient to maintain said container at atemperature below the melting point of aluminum, cooling said materialin said atmosphere of argon to a temperature below that of the meltingpoint of aluminum thereby forming a structure comprising an aluminumcontainer and a solid core member of said material, removing saidstructure from said mold assembly, and working said structure into wire,whereby spaces formed during the cooling of said assembly, between theoutside surface of said core member and the inside surface of saidcontainer, are substantially eliminated and an aluminum-clad barium wireis provided which is substantially free of entrapped gases.

References Cited in the le of this patent UNITED STATES PATENTS1,682,590 Austin Aug. 28, 1928 2,100,257 Larson Nov. 23, 1937 2,100,746Miller et al Nov. 30, 1937 2,329,317 Atlee Sept. 14, 1943 2,624,450Britten et al. Ian. 6, 1953 'FOREIGN PATENTS 567,291 Great Britain Feb.7, 1945

4. A METHOD OF MAKING AN ALUMINUM-CLAD CORE STRUCTURE HAVING A COREMATERIAL INCLUDING A METAL SELECTED FROM THE CLASS CONSISTING OFALKALINE EARTH METALS AND ALKALINE EARTH METAL ALLOYS AND CHARACTERIZEDIN BEING REACTIVE WITH THE ORDINARY ATMOSPHERE, AND COMPRISING THEOPERATIONS OF HEATING SAID MATERIAL TO A TEMPERATURE BELOW THE MELTINGPOINT THEREOF AND IN AN ATMOSPHERE AT BELOW ORDINARY ATMOSPHERICPRESSURE WHEREBY SAID MATERIAL IS SUBSTANTIALLY DEGASSED, FURTHERHEATING SAID MATERIAL IN AN ATMOSPHERE OF AN INERT GAS AT A PRESSURE ATLEAST AS HIGH AS THAT OF THE VAPOR PRESSURE OF SAID MATERIAL AND TO ATEMPERATURE SUFFICIENT TO MELT SAID MATERIAL WHEREBY SAID MATERIAL ISMELTED WHILE EVAPORATION OF SAID MATERIAL IS REDUCED, STILL FURTHERHEATING SAID MATERIAL IN SAID LAST NAMED ATMOSPHERE FOR ACHIEVING APOURING TEMPERATURE OF SAID MATERIAL WHICH IS ABOVE THE MELTING POINTTHEREOF, POURING SAID MATERIAL IN AN INERT GAS AT SAID LAST NAMEDPRESSURE INTO AN ALUMINUM CONTAINER WHICH HAS A MELTING POINT BELOW SAIDMELTING POINT WHILE DRAWING HEAT FROM THE INSIDE SURFACE OF SAIDCONTAINER THROUGH SAID CONTAINER TO MAINTAIN SAID INSIDE SURFACE AT ATEMPERATURE BELOW THE MELTING POINT THEREOF AND THEREBY PREVENTING ANYALLOYING BETWEEN SAID CORE MATERIAL AND SAID CONTAINER, COOLING SAIDMATERIAL IN AN INERT GAS TO A TEMPERATURE BELOW THE MELTING POINT OFSAID CONTAINER, AND MECHANICALLY WORKING SAID CONTAINER TO REMOVE ANYGASES ENTRAPPED THEREIN WITH SAID CORE MATERIAL AND TO CLOSE AN OPENINGIN SAID CONTAINER, WHEREBY AN ALUMINUM-CLAD CORE STRUCTURE IS PROVIDEDHAVING A SUBSTANTIAL FREEDOM FROM ENTRAPPED GASES.